SHALLOW WATER PLATFORM

Abstract

A device is described for hull in all essentials symmetrical about a centre axis and arranged to be accommodated resting on a seabed, where the hull is provided with several support means which in an active position protrudes downwards from a hull bottom and is arranged to maintain the centre axis of the hull in a specified position by each support mean being arranged for vertical displacement in relation to one or both of the hull and the seabed. It is also described a method of positioning a hull in all essentials symmetrical about a centre axis on a seabed.

Description

The invention concerns a hull that is generally symmetrical about a centre axis and arranged to be placed stationary and resting on a seabed. The invention also concerns a method of positioning such a hull on a seabed.

Platforms that are used e.g. for exploration, production and storage of oil and gas in/from submarine structures and are floating both during transportation and operation, may also be used when resting on the seabed in shallow waters. In particular, platforms having a circular, or substantially circular, horizontal cross-section (e.g. of the type known from the applicant's own patent publication WO02090177), where the buoyancy volume is rather large and the platform therefore can be submerged a relatively great amount without having too small a freeboard, are particularly suitable.

The disadvantage of having platforms of this type rest on the seabed is that the platform might assume an inclined position as its orientation is depending on the seabed conditions. If this is to be avoided, the seabed must be prepared, for example by filling and levelling, an operation which is time consuming and costly.

So-called jackup platforms, which in an operative position are resting on vertically displaceable legs and where the entire platform hull is above sea level, are not dependent of such demanding and costly preparation of the surface they will be placed upon, by that very fact that the position of the platform can be adjusted with individual regulation of each leg by means of built-in jackup mechanisms. The disadvantage with this type of platform is its low load-carrying capacity during transportation in a floating condition, and the shape of the hull and the positioning of the legs makes is vulnerable for ice, such that it is not suited for use for example in arctic regions, with its combination of demanding ice conditions, shallow waters and rough seas during bad weather.

The object of the invention is to mitigate or to reduce at least one of the disadvantages with the prior art, or at least provide a useful alternative to the prior art.

The object is achieved by features indicated in the description below and in the following patent claims.

It is provided a floatable hull which is symmetrical or essentially symmetrical about a vertical centre axis, for example cylindrical or polygonal, and is provided with a plurality of support means that at least in an operative position protrudes downwards from a hull bottom and that during lowering of the hull towards a seabed is arranged to provide an adjustable support of the hull relative to the seabed.

The support is provided by the support means resting against the seabed or penetrating into uncompacted material below the seabed, as the support means are pressed and/or sucked down into the uncompacted material until sufficient friction force between the uncompacted material and the support means is provided for stable support of the hull.

In the following, the terms “suction foot/feet” are used for the support means of the type that in principle works as in a per se known suction anchor, as the support means are formed as a cup with a downward facing open end, and where the cup is connected to an evacuation system that can provide an underpressure inside the cup such that it is sucked down into the seabed on which it is resting. Both water and uncompacted material can in principle be evacuated from the vessel, in order for the suction foot to be brought sufficiently down into the uncompacted material. It is obvious that the penetration is also affected by the mass of the hull and the buoyancy of the hull, i.e. the net downward pressure of the hull.

The expression “support leg” refers to a support means configured to be resting on a supporting surface, for example the seabed or on prepared foundation materials, without penetrating much into the surface. Typically, a lower part of the support means in this shape will have an expanded, downward facing end surface.

The support means can be formed as vertical columns, or similar. The adjustment can be provided by the support means being formed as suction feet which by differentiated internal underpressure in the suction feet are pressed into the uncompacted material below the seabed. Alternatively, the support means can be formed as support legs resting on the seabed. When the support means are formed as support legs arranged for contact with the seabed, the support means are preferably arranged to vertical displacement relative to the bottom of the hull. Vertical displacement can also be applied when the support means are formed as suction feet. Each vertically displaceable support means comprises means for individual vertical displacement. Thereby, the support means can be adjusted individually, and the hull can thereby be supportedly arranged against the seabed with the vertical centre axis independent of the slope and evenness of the seabed.

In a first aspect, the invention relates more specifically to a hull device, essentially symmetrical about a centre axis and arranged such that a hull side exhibits a predetermined freeboard, characterized in that the hull is provided with several support means which in an active position project downwards from a hull bottom and are arranged to maintain the centre axis of the hull in a specified position by each support means being arranged for vertical displacement in relation to one or both of the hull and the seabed.

The support means can be provided with suction feet arranged to be pressed into uncompacted material below the seabed, and which is rigidly connected to the hull bottom.

The support means can be provided with suction feet configured to be pressed into uncompacted material below the seabed, or may be provided with support legs arranged to about against the seabed, and which are individually vertically displaceable in relation to the hull bottom.

The support means may in a retracted position be at least partly accommodated by respective pockets inside the hull. The outer shape of the hull is thereby maintained, as the hull and the support means show less vulnerability against harmful impacts of ice.

The vertical displacement of each support means is provided by one or more actuators associated with the hull and the support means.

The hull may comprise a ballasting system arranged to be able to lower the hull until a hull side exhibits a specified freeboard.

In a second aspect the invention relates to more specifically a method for positioning a hull, that is essentially symmetrical about a centre axis, on a seabed, characterized in that the method comprises the steps of:

• • a) arranging the hull floating on a specified location above the seabed; ballasting the hull such that a hull side exhibits a predetermined freeboard; and
  • b) displacing several support means vertically downwards to contact with the seabed, possibly into uncompacted material below the seabed to obtain stable support of the hull.

The method may comprise the further step of:

• • d1) displacing the support means provided with suction feet in relation to the seabed during ballasting of the hull by applying a differentiated underpressure to the suction feet.

Alternatively, the method may comprise the further step of:

• • d2) displacing the support means provided with support legs in relation to the hull bottom by means of one or more actuators.

The method may comprise the further step of:

• • e) locking each of the support means relative the hull by means of the respective actuator.

In the following, an example of a preferred embodiment is described, illustrated in the attached drawings, where:

FIG. 1 shows in perspective, at an angle seen from below, a principle drawing of a hull according to a first embodiment of the invention, as the hull is provided with fixed support means in the form of suction feet;

FIG. 2 shows in perspective, at an angle seen from below, a principle drawing of a hull according to a first embodiment of the invention, as the hull is provided with vertically displaceable support means in the form of suction feet;

FIG. 3 shows in a side view a principal sketch of a hull according to prior art arranged on a prepared surface on a sloping seabed;

FIG. 4 shows in a side view a principal sketch of the hull according to a first embodiment of the invention supported on a sloping seabed by means of supporting means in the form of fixed suction feet pressed partly into a uncompacted material below the seabed; and

FIG. 5 shows in a side view a principal sketch of the hull according to a first embodiment of the invention supported on a sloping seabed by means of supporting means in the form of vertically displaceable support feet resting against the seabed.

In the figures, the reference number 1 indicates a cylindrical hull with a vertical centre axis 11 in its operative position and partly with an inclined outwardly sloping upper portion 1a. Even though the hull 1 here is shown symmetrically, the invention also covers other hull shapes which are essentially symmetrical about a vertical centre axis, for example a polygonal hull. Due to stability considerations during the floating condition of the hull 1 the hull 1 is provided with a hull skirt 13 of a per se known type.

In a first embodiment, illustrated in FIGS. 1 and 4, the hull 1 is provided with several fixedly arranged support means 17 protruding downwards from a hull bottom 12. In this embodiment the support means 17 are provided with suction feet 17′ which by means of a vacuum plant (not shown) a differentiated underpressure may be applied such that the suction feet 17′ by means of the internal underpressure are sucked down into the uncompacted material 31 below a seabed 3. By controlling the underpressure individually for each suction foot 17′, the hull 1 can be levelled even if the seabed 3 is sloping.

In a second embodiment, illustrated in FIGS. 2 and 5, the hull 1 is provided with several pockets 15, each accommodating a support means 17 provided with support legs 17″ arranged to rest against the seabed 3 and which is vertically displaceable between an inactive position essentially retracted into the hull 1 and an active position partly protruding downwards from the hull bottom 12 through respective openings 121 in the hull bottom 12. The vertical displacement of each of the support means 17 is provided by means of one or more actuators 16, shown here schematically as one actuator, as the actuators form a connection between the hull 1 and the support means 17.

The support means 17 with suction feet 17′ according to the first embodiment may be combined with the vertical displacement of the support means 17 according to the second embodiment. This combination will be able to provide better certainty that the hull 1 being capable of being levelled also when the condition of the uncompacted material obstructs or restrains the penetration of the suction feet 17′.

Even though the embodiment according to FIGS. 1 and 4 and according to FIGS. 2 and 5 is shown with three support means 17, the effect of the invention can in principle be achieved by means of only two support means 17, as a portion of the periphery of the hull bottom 12 can constitute a support means abutment portion which rests against the highest area of the portion of the seabed 3 on which the hull 1 is to be supported, while the adjustment of the inclination of the hull 1 is provided by the two support means 17 by suction or vertical displacement or a combination thereof.

The hull 1 may be adapted to the condition of the seabed 3 by providing the hull with support means 17 of the type most suitable for the existing conditions. This is especially obvious in the embodiment where the support means 17 are vertically displaceable.

The hull 1 is in a per se known way provided with a ballasting system 14 for trimming the hull 1, including giving the hull 1 sufficient stability in a floating condition during transportation between locations, and to submerge the hull 1 until a hull side 18 exhibits a specified freeboard 181 in an operative condition where the hull 1 is to be supported by the seabed 3.

Devices (not shown) for operation of the actuators 16 and pumps etc. (not shown) for providing a suction effect when the suction feet 17′ are placed or released, is not shown. These can be arranged in the hull 1, but they can also be arranged on tender vessels (not shown) if for example there is a lack of space in the hull 1.

Claims

1. A hull substantially symmetrical about a centre axis and configured to be arranged such that a hull side exhibits a predetermined freeboard, the hull comprising:

several support members that in an active position protrude downwards from a hull bottom and are arranged to maintain the centre axis of the hull in a specified position, each support member being arranged for vertical displacement in relation to one or both of the hull and a seabed.

2. The device of claim 1, wherein the support members are provided with suction feet arranged to be pressed into uncompacted material below the seabed, and which is rigidly connected to the hull bottom.

3. The device of claim 1, wherein the support members are provided with suction feet arranged to be pressed into uncompacted material below the seabed, or provided with support legs arranged to contact the seabed, and which are individually vertically displaceable in relation to the hull bottom.

4. The device of claim 3, wherein the support members in a pulled up position are at least partly accommodated by respective pockets inside the hull.

5. The device of claim 1, wherein the vertical displacement of each support member is provided by one or more actuators connected to the hull and the support members.

6. The device of claim 1, wherein the hull comprises a ballasting system arranged to be able to submerge the hull until a hull side exhibits a predetermined freeboard.

7. A method of positioning a hull, that is essentially symmetrical about a centre axis, on a seabed, comprising: arranging the hull floating on a specified location above the seabed;

ballasting the hull such that a hull side exhibits a predetermined freeboard; and

displacing several support members vertically downwards and into abutment with the seabed, potentially into uncompacted material below the seabed to stable support of the hull.

8. The method of claim 7, wherein the method further comprises:

displacing the support members provided with suction feet in relation to the seabed during ballasting of the hull by applying the suction feet a differentiated underpressure.

9. The method of claim 7, wherein the method further comprises:

displacing the support members provided with suction feet in relation to the hull bottom by means of one or more actuators.

10. The method of claim 8, wherein the method further comprises:

locking each of the support members relative the hull by means of the respective actuator.